Lightguide optical combiner for head wearable display

ABSTRACT

An eyepiece for a head wearable display includes a lightguide component for guiding display light and emitting the display light along at a viewing region. The light guide component includes an input surface oriented to receive the display light into the lightguide component at the peripheral location, a first folding surface disposed to reflect the display light received through the input surface, a second folding surface disposed to reflect the display light received from the first folding surface, an eye-ward facing surface disposed opposite to the second folding surface to reflect the display light received from the second folding surface, and a curved reflective surface having reflective optical power disposed at the viewing region to receive the display light reflected from the eye-ward facing surface and to reflect the display light for emission out through the eye-ward facing surface.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 14/271,083, filed May 6, 2014, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to eyepieces for head wearable displays.

BACKGROUND INFORMATION

A head mounted display (“HMD”) or head wearable display is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to create a magnified virtual image placed a few meters in front of the user. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD typically includes some form of see-through eyepiece and can serve as the hardware platform for realizing augmented reality. With augmented reality the viewer's image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”).

HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are limited due to the cost, size, weight, field of view, eye box, and efficiency of conventional optical systems used to implemented existing HMDs.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described.

FIGS. 1A and 1B are cross-sectional views of an eyepiece for use with a head wearable display, in accordance with an embodiment of the disclosure.

FIGS. 2A and 2B are cross-sectional views of an eyepiece for use with a head wearable display, in accordance with another embodiment of the disclosure.

FIG. 3A is a cross-sectional view of an eyepiece including an alcove notched into a lightguide component to accommodate a camera module within a temple housing, in accordance with an embodiment of the disclosure.

FIG. 3B is a perspective view of an eyepiece including an alcove notched into a lightguide component to accommodate a camera module within a temple housing, in accordance with an embodiment of the disclosure.

FIGS. 4A and 4B illustrate a demonstrative head wearable display using an eyepiece including a lightguide optical combiner, in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Embodiments of a system and apparatus that integrates a total internal reflection (“TIR”) based lightguide and optical combiner into an eyepiece for a head wearable display are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIGS. 1A and 1B are cross-sectional views of an eyepiece 100 for use with a head wearable display, in accordance with an embodiment of the disclosure. The illustrated embodiment of eyepiece 100 includes a lightguide component 105 and a see-through add-on component 110. The illustrated embodiment of lightguide component 105 includes an input surface 115, a first folding surface 120, a second folding surface 125, an eye-ward facing surface 130, a curved reflective surface 135, and an end surface 140. See-through add-on component 110 includes an interface surface 145, an external scene facing surface 147, and an end surface 150. In the illustrated embodiment a diffusor 155 is coated over end surfaces 140 and 150.

Display source 160 is aligned to inject display light 165 into lightguide component 105 through input surface 115. Display source 160 is located at a peripheral location (proximal end), which is offset from a viewing region 170 near the distal end of eyepiece 100. Display light 165 is emitted from lightguide component 105 in viewing region 170 along an eye-ward direction for viewing by a user. As such, lightguide component 105 operates as a lightguide that transports display light 165 from a peripheral location outside of the user's center of vision to viewing region 170 located nearer to the user's central or foveal vision.

Eyepiece 100 can be implemented in a see-through or non-see-through version, and as such see-through add-on component 110 is an optional component. In see-through embodiments, curved reflective surface 135 is layered with a partially reflective element (e.g., beam splitter coating, polarizing beam splitter coating, diffractive reflector, etc.). The partial reflectivity of curved reflective surface 135 permits ambient scene light 175 to pass through viewing region 170 and combine with display light 165 emitted out through viewing region 170. When indexed matched to lightguide component 105, see-through add-on component 110 defeats the optical power associated with curved reflective surfaced 135 for the ambient scene light 175 passing through. Accordingly, interface surface 145 of see-through add-on component 110 has a size and curvature that mates to and complements the curvature of curved reflective surface 135 of lightguide component 105. Correspondingly, external scene facing surface 147 is complementary to eye-ward facing surface 130 in viewing region 170 to ensure ambient scene light 175 experiences substantially no optical power.

In non-see-through embodiments, curved reflective surface 135 may implemented as a mirror surface with or without add-on component 110 according to industrial design choice.

In one embodiment, lightguide component 105 and add-on component 110 are fabricated as two independent pieces that are bonded together along interface surface 145 and curved reflective surface 135 using a clear adhesive. Lightguide component 105 and add-on component 110 may be fabricated of two different materials having the same index of refraction, or both of the same material. For example, lightguide component 105 and add-on component 110 may be fabricated of optical grade plastic (e.g., Zeonex E-48R), glass, or otherwise. In one embodiment, the components are injection molded to shape, processed to add various optical coatings/layers discussed below, and then bonded together along interface surface 145 and curved reflective surface 135. In one embodiment, lightguide component 105 and add-on component 110 are fabricated of a material having a higher index of refraction than air to induce total interface reflection (“TIR”) at first folding surface 120, second folding surface 125, and eye-ward facing surface 130.

In an embodiment wherein curved reflective surface 135 is coated with a partially reflective material, the splitting ratio may be selected according to design needs, but in one embodiment may be implemented as a 50/50 beam splitter. In embodiments where curved reflective surface 135 is implemented using a polarizing beam splitter (“PBS”), display source 160 would output polarized light with a polarization selected to substantially reflect off of the PBS material. A PBS design can serve to increase the efficiency of the optical system. For example, LCD or liquid crystal on silicon (“LCoS”) are example display technologies that output polarized light. Of course, external polarizing films may be used in connection with other non-polarized display technologies. When operating with polarized light, it can be beneficial to use low stress materials to reduce the influence of birefringence on the optical design. Accordingly, in some embodiments, lightguide component 105 may be fabricated of low stress plastics, glass, or other low stress optical grade materials.

In see-through embodiments, lightguide component 105 and add-on component 110 are fabricated of optically transmissive materials (e.g., clear plastic) that permit at least a portion of external scene light 175 to pass through viewing region 170 to the user's eye. As such, eyepiece 100 operates as an optical combiner combining external scene light 175 with display light 165 emitted out through eye-ward facing surface 130 in viewing region 170 along an eye-ward direction into the eye. In this way, eyepiece 100 is capable of displaying an augmented reality to the user.

During operation, display source 160 emits display light 165 from a peripheral location offset from viewing region 170 into lightguide component 105. Display source 120 may be implemented using a variety of different display technologies including LCD displays, LCoS displays, organic light emitting diode (“OLED”) displays, or otherwise. Display light 165 may include computer generated images.

Display light 165 is incident into lightguide component 105 through input surface 115. Input surface 115 is a curved surface with optical power. In one embodiment, input surface 115 is a cylindrical lensing surface that in connection with the other lensing surfaces can be adjusted to correct aberrations and distortions in the optical system. In the illustrated embodiment, input surface 115 is a cylindrical convex surface (as viewed from display source 160) having its center axis of symmetry in the plane of the page running parallel to the line drawn as input surface 115.

After display light 165 enters into lightguide component 105 through input surface 115, it is incident upon first folding surface 120, which is disposed proximate to input surface 115. First folding surface 120 operates to reflect display light 165 towards second folding surface 125. In the illustrated embodiment, first folding surface 120 is also a curved surface with reflective optical power. For example, first folding surface 120 may be implemented as a cylindrical surface with optical power to aid in correction of aberrations and distortions in the optical system. In the illustrated embodiment, first folding surface 120 is a cylindrical concave surface (as viewed external to lightguide component 105) having its center axis of symmetry in the plane of the page running parallel to the line drawn as first folding surface 120.

After folding (e.g., reflecting) and lensing display light 165 at first folding surface 120, display light 165 is directed towards second folding surface 125 where display light 125 is once again redirected back across lightguide component 105 to eye-ward facing surface 130. In the illustrated embodiment, second folding surface 125 is a planar surface without optical power; however, in other embodiments, second folding surface 125 may also have curvature to impart optical power.

Display light 165 incident upon eye-ward facing surface 130 for the first time is reflected to curved reflective surface 135. In one embodiment, eye-ward facing surface 130 is a planar surface without optical power that is opposite, but parallel to second folding surface 125. Eye-ward facing surface 130 and first folding surface 120 are non-coplanar surfaces off-set from each other.

Curved reflective surface 135 is implemented as an off-axis aspheric lens that provides reflective optical power to collimate or nearly collimate display light 165 emitted from eyepiece 100. For example, display light 165 may be virtually displaced to appear to 2 m to 3 m in front of the user. Of course other amounts of collimation may be implemented. After reflection off of curved reflective surface 135, display light 165 is directed back to eye-ward facing surface 130 in viewing region 170 where display light 165 is emitted out of eyepiece 100 along an eye-ward direction. The second encounter with eye-ward facing surface 130 does not result in TIR, since the angle of incidence is steeper than the required critical angle for TIR.

Eyepiece 100 provides a relatively large eye box (e.g., 8.5 mm horizontal and 6.2 mm vertical) due to its inherent design. This large eye box is due in part to the close proximity of curved reflective surface 135 to the user's eye. Additionally, the relatively shallow oblique angle of curved reflective surface 135 projects a large horizontal eye box area onto eye-ward facing surface 130 in viewing region 170, which also contributes to the eye box size. A large eye box accommodates larger inter-pupillary deviations, thereby providing a larger cross-section of the population with an improved user experience.

In one embodiment, first folding surface 120, second folding surface 125, and eye-ward facing surface 130 are clear surfaces that reflect display light 165 via TIR and careful design control over the incident angles of the light path followed by display light 165. By using TIR for the reflections off of the folding surfaces, eyepiece 100 achieves desirable industrial design characteristics, since eyepiece 100 will appear as a clear eyepiece to external observers. Furthermore, TIR reflections are highly efficient. In an example where curved reflective surface 135 is a 50/50 beam splitter, embodiments of eyepiece 100 can approach near 50% efficiency. In other embodiments, first folding surface 120 and second folding surface 125 may be coated with reflecting films to reflect display light 165 without need of TIR. FIG. 1B illustrates example optical paths through eyepiece 100 by a number of ray trace bundles of display light 165 output from display source 160.

In the illustrated embodiment, diffusor 155 is coated over the distal ends 140 and 150 of lightguide component 105 and add-on component 110, respectively. Diffusor 155 operates to absorb incident light to reduce deleterious back reflections. Diffusor 155 may be implemented as a dark diffusive paint (e.g., matte black paint), and in some embodiments, further includes an anti-reflective coating under the dark diffusive paint. In one embodiment, diffusor 155 includes an opening to permit a portion of display light 165 to bleed out the distal end of eyepiece 200 as a sort of indicator light. The indicator light provides third persons a visual cue that display source 160 is turned on. In one embodiment, the opening may be an image or logo stenciled into the dark diffusive paint and may include a transparent diffusive element under the stenciled image/logo to diffuse the display light emitted as a visual cue.

FIGS. 2A and 2B are cross-sectional views of an eyepiece 200 for use with a head wearable display, in accordance with another embodiment of the disclosure. The illustrated embodiment of eyepiece 200 includes a lightguide component 205 and a see-through add-on component 210. The illustrated embodiment of lightguide component 205 includes an input surface 215, notch surfaces 217 and 218, a first folding surface 220, a second folding surface 225, an eye-ward facing surface 230, curved reflective surface 235, and an end surface 240. See-through add-on component 210 includes an interface surface 245, an external scene facing surface 247, and an end surface 250. In the illustrated embodiment a diffusor 255 is coated over end surfaces 240 and 250.

Eyepiece 200 is similar to eyepiece 100 except that notch surfaces 217 and 218 proximal to input surface 215 form an alcove 219 suitably sized to house a camera module or other optical/electrical systems. Furthermore, first folding surface 220 is tilted towards display source 160 and lengthened to extend between (and directly interface with) input surface 215 and eye-ward facing surface 230.

FIGS. 3A and 3B illustrate an example housing configuration for eyepiece 200, in accordance with an embodiment of the disclosure. FIG. 3A is a cross-sectional view while FIG. 3B is a perspective view of the same. As illustrated, the proximal end of eyepiece 200 inserts into a housing 305. Housing 305 is shaped for mounting to a temple region of an eyewear-like frame for wearing on a head of user (e.g., see FIGS. 4A and 4B). Frame 305 positions display source 160 peripherally to the user's central vision. Further as illustrated, alcove 219 provides a convenient location for additional circuitry or optical components, such as for example, a forward facing camera module 310.

In one embodiment, eyepiece 200 delivers display light 265 with a 15 degree field of view having a 16:9 aspect ratio (e.g., 13 degree horizontal and 7.35 degrees vertical) and a resolution of approximately 4 arc mins based upon display source 160 having a 640×360 pixel display and 7.5 um pixel size. Additional design specification of such an embodiment include an eye relief (D1) of 18 mm and approximate lightguide component dimensions including: D2=7.2 mm, D3=25 mm, D4=15 mm, and rectangular cross sectional dimensions along line A-A′ of 7.2 mm×10 mm. Eyepiece 200 is also capable of providing a relatively large eye box (e.g., 8.5 mm horizontal by 6.2 mm vertical) for similar reasons as discussed above in connection with eyepiece 100. Of course, these dimensions are merely demonstrative and alternative dimensions may be implemented. In one embodiment, curved reflective surface 235 is an off-axis asphere with a sag equation:

${{Z(r)} = {{\frac{r^{2}}{R}\frac{1}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( {r\text{/}R} \right)^{2}}}}} + {\beta_{3}r^{3}} + {\beta_{4}r^{4}}}},$

where R=−81.62, k=−3.63, β₃=−5.00 E−05, and β₄=−3.81 E−08. In one embodiment, input surface 215 is a regular cylinder with a radius of R=−6.502 having an orientation that is similar to that described above in connection with input surface 115. In the illustrated embodiment, the local coordinate system of curved reflective surface 235 for the sag equation provided above is offset compared to the center of viewing region 270 by −43.52 mm in X, 2.4 mm in Y, and 11.32 mm in Z. In this embodiment, the local coordinate system of curved reflective surface 235 is further rotated in the Y-Z plane by −3.7 degrees, and in the X-Z plane −8.95 degrees. In one embodiment, first folding surface 220 is an off axis toroid with a sag equation:

${Z(y)} = {{\frac{y^{2}}{R}\frac{1}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( {y\text{/}R} \right)^{2}}}}} + {\alpha \; y}}$

where R=−7.113, a=0.061, k=0.00, and a radius of rotation of 1468. In this embodiment, the center of the radius of rotation is offset −453.77 mm in X, 0 mm in Y and 1401.06 mm in Z relative to the center of the viewing region 270. In other embodiments, first folding surface 220 is a cylinder having an orientation that is similar to that described above in connection with first folding surface 120. In one embodiment, input surface 215 is a cylinder with a convex radius of −7.175 mm and an angle of 70 degrees to eye-ward facing surface 230. Of course, these curvatures, positions, and angles are merely demonstrative and alternative curvatures, positions, and angles may be implemented.

FIGS. 4A and 4B illustrate a monocular head wearable display 400 using a eyepiece 401, in accordance with an embodiment of the disclosure. FIG. 4A is a perspective view of head wearable display 400, while FIG. 4B is a top view of the same. Eyepiece 401 may be implemented with embodiments of eyepieces 100 or 200 as discussed above. Eyepiece 401 is mounted to a frame assembly, which includes a nose bridge 405, left ear arm 410, and right ear arm 415. Housings 420 and 425 may contain various electronics including a microprocessor, interfaces, one or more wireless transceivers, a battery, a camera, a speaker, etc. Although FIGS. 4A and 4B illustrate a monocular embodiment, head wearable display 400 may also be implemented as a binocular display with two eyepieces 401 each aligned with a respective eye of the user when display 400 is worn.

Eyepiece 401 is secured into an eye glass arrangement or head wearable display that can be worn on the head of a user. The left and right ear arms 410 and 415 rest over the user's ears while nose bridge 405 rests over the user's nose. The frame assembly is shaped and sized to position the viewing region in front of an eye of the user. Other frame assemblies having other shapes may be used (e.g., traditional eyeglasses frame, a single contiguous headset member, a headband, goggles type eyewear, etc.).

The illustrated embodiment of head wearable display 400 is capable of displaying an augmented reality to the user. A see-through embodiment permits the user to see a real world image via ambient scene light 175. Left and right (binocular embodiment) display light 480 may be generated by display sources 160 mounted in peripheral corners outside the user's central vision. Display light 480 is seen by the user as a virtual image superimposed over ambient scene light 175 as an augmented reality. In some embodiments, ambient scene light 175 may be fully, partially, or selectively blocked to provide sun shading characteristics and increase the contrast of display light 480.

The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. 

1. An eyepiece assembly for a wearable display, the eyepiece assembly comprising: a lightguide component configured to guide display light received from a display source and to emit the display light and an ambient light scene to a user of the wearable display along a viewing region, the light guide component including: an input surface oriented to receive the display light into the lightguide component from the display source; a first folding surface disposed to receive the display light from the input surface without reflection and to reflect the received display light; a second folding surface disposed to receive the reflected display light directly from the first folding surface and to generate a secondary reflection of the reflected display light, the second folding surface being a planar surface; and an eye-ward facing surface disposed to face the second folding surface to receive light from the secondary reflection, wherein the eye-ward facing surface includes the viewing region, the viewing region being positioned to emit the received light and the ambient light scene out of the lightguide component toward an eye of the user; and a camera module having a lens facing outward from the lightguide component away from the eye-ward facing surface, the camera module being disposed adjacent to at least one of the input surface and the second folding surface.
 2. The eyepiece assembly of claim 1, wherein the lightguide component further includes: a first notch surface that interfaces by contact with the input surface; and a second notch surface that interfaces by contact with the second folding surface, the second notch surface being non-coplanar with the first notch surface; wherein the first and second notch surfaces form an alcove in the lightguide component adjacent to the input surface, and the camera module is disposed in the alcove.
 3. The eyepiece assembly of claim 1, further comprising the display source.
 4. The eyepiece assembly of claim 3, further comprising a frame assembly to support the lightguide component and the camera module for wearing on a head of the user so that the viewing region is positioned in front of the eye of the user.
 5. The eyepiece assembly of claim 3, further comprising a frame assembly to support the lightguide component, the camera module and the display source for wearing on a head of the user so that the viewing region is positioned in front of the eye of the user, wherein the frame assembly is configured to position the display source peripherally to the user's central vision.
 6. The eyepiece assembly of claim 1, wherein the viewing region is positioned to emit the received light and the ambient light scene out of the lightguide component so that the received light is presented as a virtual image superimposed over the ambient light scene as an augmented reality scene.
 7. The eyepiece assembly of claim 6, wherein the eyepiece assembly is configured to increase contrast of the received light by fully, partially or selectively blocking the ambient light scene.
 8. The eyepiece assembly of claim 1, further comprising: a curved reflective surface having reflective optical power, the curved reflective surface disposed to receive the light from the secondary reflection, to collimate the received light and to direct the collimated light to the viewing region of the eye-ward facing surface; wherein the eye-ward facing surface is disposed to receive the collimated light from the secondary reflection and the viewing region is positioned to emit the collimated light and the ambient light scene out of the lightguide component toward the eye of the user.
 9. The eyepiece assembly of claim 8, wherein the curved reflective surface comprises a partially reflective surface that is configured to partially reflect the light received from the secondary reflection, to transmit the ambient scene light through the viewing region, and to optically combine the ambient scene light with the light received from the secondary reflection.
 10. The eyepiece assembly of claim 8, further comprising: a see-through add-on component mounted to the lightguide component along the curved reflective surface, the see-through add-on component being formed of a material having an index of refraction substantially equivalent to that of the lightguide component, wherein the see-through add-on component is at least partially transparent to the ambient scene.
 11. The eyepiece assembly of claim 10, wherein the see-through add-on component includes: an interface surface having a size and a curvature that mates to and complements the curved reflective surface of the lightguide component; and an external scene facing surface having an alignment such that the ambient scene light that passes through the see-through add-on component and the lightguide in the viewing region experiences substantially no optical power.
 12. The eyepiece assembly of claim 1, wherein the first folding surface, the second folding surface, and the eye-ward facing surface are clear surfaces that are oriented relative to each other to reflect the display light via total internal reflection.
 13. A wearable display system configured to display imagery to a user, the wearable display comprising: a display source configured to generate display light; a lightguide component configured to guide the display light received from the display source and to emit the display light and an ambient light scene to a user of the wearable display system along a viewing region, the light guide component including: an input surface oriented to receive the display light into the lightguide component from the display source; a first folding surface disposed to receive the display light from the input surface without reflection and to reflect the received display light; a second folding surface disposed to receive the reflected display light directly from the first folding surface and to generate a secondary reflection of the reflected display light, the second folding surface being a planar surface; and an eye-ward facing surface disposed to face the second folding surface to receive light from the secondary reflection, wherein the eye-ward facing surface includes the viewing region, the viewing region being positioned to emit the received light and the ambient light scene out of the lightguide component toward an eye of the user; a camera module having a lens facing outward from the lightguide component away from the eye-ward facing surface, the camera module being disposed adjacent to at least one of the input surface and the second folding surface; and a housing receiving the camera module and at least a portion of the lightguide component therein, the housing being configured for wearing on a head of the user.
 14. The wearable display system of claim 13, wherein the housing further includes at least one of a microprocessor, a wireless transceiver, a battery, or a speaker.
 15. The wearable display system of claim 13, wherein the lens of the camera module is arranged to view the ambient light scene.
 16. The wearable display system of claim 13, wherein a front surface of the lens of the camera module is coplanar with the second folding surface.
 17. The wearable display system of claim 13, wherein: the wearable display system is a monocular wearable display system; and the viewing region of the monocular wearable display system is positioned to emit the received light and the ambient light scene so that the received light is presented as a virtual image superimposed over the ambient light scene as an augmented reality scene.
 18. The wearable display system of claim 13, wherein: the wearable display system is a binocular wearable display system having a pair of lightguide components; and the viewing region of each lightguide component is positioned to emit the received light and the ambient light scene out toward a respective eye of the user so that the received light is presented as a virtual image superimposed over the ambient light scene as an augmented reality scene.
 19. The wearable display system of claim 13, wherein the camera module is disposed in an alcove of the lightguide component.
 20. The wearable display system of claim 19, wherein the alcove is formed by a first notch surface that interfaces by contact with the input surface and a second notch surface that interfaces by contact with the second folding surface, the second notch surface being non-coplanar with the first notch surface. 